1. Field of the Invention
The present invention relates to an aperture antenna such as a horn antenna or a reflector antenna and, more specifically, to an aperture antenna having nonuniform resistive properties in one or more dimensions.
2. Description of the Related Art
The present invention is particularly well suited to aperture antennas which have waveguides as antenna feed lines, such as horn antennas and reflector antennas. Examples of horn antennas include rectangular horns such as E and H plane sectoral and pyramidal horns, circular horns, and other types such as diagonal horns. Examples of reflector antennas include circular, elliptical, parabolic, hyperbolic and cosecant squared antennas.
The waveguide is electromagnetically coupled at one end (a circuit end) to transmit/receive circuitry and at the other end (antenna end) to the aperture antenna to transmit and/or receive electromagnetic signals. The waveguide functions essentially as a transmission line antenna feed, with the antenna and free space serving as the load.
Ideally, all of the electromagnetic energy input into the circuit end of the waveguide by the transmitter is outputted at the antenna end, and the antenna radiates this energy into a desired region in space (target zone) along a longitudinal axis of the antenna corresponding to the main beam or lobe of the antenna. Similarly, external electromagnetic signals propagating into the antenna along the longitudinal axis of the antenna, i.e., from the target zone, ideally are directed to the antenna end of the waveguide by the reflective surface of the antenna so that essentially all of the energy of the received signal is directed to the receiver at the circuit end of the waveguide. In practical antenna systems, however, some portion of the energy being transmitted or received is effectively lost. This effective loss can be attributable to a number of factors related to the design of the antenna.
A number of antenna parameters are used to characterize the design of a given antenna, the selection of which may strongly influence the operation of the system employing the antenna. Examples of these antenna parameters are the voltage standing wave ratio ("VSWR"), side and back lobe structure, and gain.
As with any conventional transmission line, maximum energy transfer occurs when the impedance of the transmission line is equal to that of the load, i.e., the impedances are matched. A poorly matched antenna will result in internal reflections of the signal in the transmission line, which reduce transmitted or received energy of the signal and dissipate a portion of the signal as unwanted thermal energy, correspondingly reducing system performance. The VSWR is the conventional measure of the impedance mismatch between an antenna and its transmission line feed. The VSWR is typically defined as the ratio of the maximum to the minimum voltage (or current) along the transmission line, and preferably has a value at or near unity (1.0). The VSWR of conventional horn antennas is largely attributable to two reflection components, one at the throat of the horn and another at the aperture of the horn. The geometric discontinuities of these locations result in these internal reflections
The side lobe structure of an aperture antenna, to a first order approximation, is attributable to diffraction of the transmitted or received electromagnetic wave caused by the abrupt termination of the metallic antenna structure at its aperture. This diffraction has the undesirable effect of spreading the signal energy across a region extending outward away from the main beam as defined by the longitudinal axis of the antenna, and outside the target zone. The signal energy transmitted to or received from the desired target zone corresponding to the longitudinal axis of the antenna is correspondingly reduced. In addition, the presence of significant side lobes increases the difficulty of discriminating signals propagating within the main beam from signals propagating off the main beam, i.e., outside the target zone.
In addition to the main beam and side lobes, a portion of the energy at the antenna will be distributed behind the antenna, more than 90.degree. away from the longitudinal axis along which main beam propagates in front of the antenna. This energy distribution is generally referred to as back lobes.
Antenna gain is a measure of the directivity of an antenna. Gain is dependent upon the geometry of the antenna and the materials from which it is made. Gain is also proportional to the square of the effective cross sectional area of the antenna. Gain may also be dependent upon phase variations across the aperture of the antenna.
Low gain and large side lobes cause a large distribution of energy in the fringe areas of the main beam. This energy is often reflected from local objects outside the target zone such as buildings, trees, and the ground plane. These reflections cause spurious signals which may result in ambiguities or false information.
A number of approaches have been proposed in the past to improve the energy distribution and beam quality of aperture antennas. For example, Sato et al. U.S. Pat. No. 3,624,655, discloses a horn antenna in which a band of dielectric material is arranged circumferentially and symmetrically with the axis of the horn on a part of the inner surface of the horn to produce a transmitted beam having an essentially flat profile. Similarly, Wong et al. U.S. Pat. No. 4,141,015, discloses a conical horn antenna having dual dielectric bands on the interior surface of the horn to cause the dominant and higher order modes of transmitted waves at a single selected frequency to be in phase and add vectorially, which results in a planar wave front. Suetaki et al. U.S. Pat. No. 3,631,504, discloses a parabolic reflector antenna having electromagnetic wave absorbing materials at the circumferential edge or aperture of the antenna. The absorbing materials prevent edge reflections and side lobe radiation associated with the antenna aperture by absorbing a portion of the energy at the peripheral edge region. This reportedly improves side and back lobe radiation.
While each of these and other past attempts to design improved aperture antennas appear to have advanced the state of the art, they generally suffer a number of drawbacks. These and other attempts to control side and back lobe radiation have relied principally on energy absorption and interference cancellation. As a result, they tend to be highly frequency specific, and dependent upon the particular absorbing material and geometries used. Since these designs use bulk dielectric materials on the interior reflective surface of the antenna, they introduce new reflective edges, reduce effective antenna area, and create a barrier to free space.
Thus, the problem of designing an aperture antenna having low side and back lobes and high gain while providing an acceptable VSWR has been an important and largely unmet objective.